This invention relates generally to radiation dosimetry techniques and is concerned particularly with dosimetry of low energy radiation. More particularly, the invention relates to dosimetry techniques for low energy beta radiation with particular emphasis directed toward tritium dosimetry. More specifically, the present invention is directed towards a dosimetry tehnique which employs the radiation-induced, thermally-activated polarization/radiation-induced, thermally-activated depolarization (RITAP/RITAD) phenomenon which occurs in dielectric materials.
The radiation-induced, thermally-activated current (RITAC) is a relatively recently discovered radiation-induced phenomenon occurring in dielectric materials, which has found useful application in radiation dosimetry techniques. This phenomenon was discovered in studies directed toward thermoluminescent dosimetry and thermally stimulated currents and, while somewhat related to these other phenomena, has been found actually to be a different and independent phenomenon in itself. The RITAC phenomenon as used herein is a general term which includes the RITAP and RITAD effects.
The RITAD phenomenon has been reported in a paper co-authored by the present inventor which appeared in Physical Review Letters, Vol. 29, No. 11, Sept. 11. 1972. The particular RIIAD phenomenon disclosed in this paper has come to be known as an external RITAD effect. In accordance with this effect and as reported in the above-mentioned paper, a stable electrical polarization is effected in a dielectric material when a RITAD dosimeter is exposed to radiation in the presence of an externally applied electric field. This polarization is proportional to the radiation dose absorbed by the dosimeter and can be subsequently read out as a function of depolarization current versus temperature as the dosimeter is gradually heated through an appropriate temperature range. The heating of the dosimeter raises the energy level of the dielectric material to the point of onset of ionic conductivity, at which point the radiation-induced polarization in the dielectric material becomes unstable and a depolarization current is generated. In the external RITAD technique the dosimeter, which includes a dielectric material disposed between polarizing electrodes, is preliminarily annealed to remove any stored energy which may be present in the dielectric material and the polarizing electrodes are grounded during cooling. A high external electric field is established across the two polarizing electrodes of the dosimeter during the exposure to radiation, consequently giving the name external RITAD, and subsequent to the exposure the polarizing electrodes are shorted through an ammeter during the readout. During the readout, the stable electrical polarization induced in the dielectric material by the radiation becomes unstable and generates a thermally-activated depolarization current as the dosimeter is heated through a characteristic temperature range and the radiation dose is determined by measuring or plotting the depolarization current versus temperature.
A related but somewhat different RITAP/RITAD effect was reported in a paper coauthored by the present inventor which paper appeared in Science, Vol. 179, pages 380-382, Jan. 26, 1973. This effect, which is referred to as the local RITAD effect, differed from the external RITAD effect in that no external electrical field was applied to the dosimeter during either radiation exposure or readout. Rather it was found that a high-temperature bias-polarization procedure produced an electric field within the material itself which produced a RITAD effect. In accordance with this technique, the polarizing electrodes of the dosimeter are tied into an electrical circuit during the annealing of the dosimeter. In this way, a high-temperature bias-polarization is induced in the dosimeter dielectric material prior to the exposure to the radiation. A high voltage is established across the electrodes as the dosimeter is heated to a high temperature, and the voltage is maintained across the electrodes while the dosimeter is cooled. The bias-poling voltage is removed after cooling and the dosimeter electrodes are shorted and grounded during exposure to the radiation and are grounded through an ammeter during readout. As a result of the bias-polarization pretreatment, the sample retains a very strong electret polarization which stable bias-polarization state produces local electric fields in the sample which give rise to the RITAD effect after irradiation, consequently giving the name local RITAD effect. During the subsequent readout following radiation exposure, the dose of radiation received by the dosimeter is again measured as the function of depolarization current versus temperature as the dosimeter is gradually heated through a characteristic temperature range. As the dosimeter reaches a sufficient temperature, it reaches the point of onset of ionic conductivity and depolarization of the established radiation-induced polarization occurs, giving the current readout. The depolarization current generated is proportional to the dose of radiation absorbed.
A better understanding of this phenomenon can be obtained from the more complete discussion of the subject contained in the two above-identified reports and the report "Radiation and Impurity-Induced Thermally Activated Charge Transport in Calcium Fluoride" by Ervin V. Podgorsak and P. R. Moran available as USAEC Technical Report C00-1105-184, which three reports are expressly incorporated herein by reference as though fully set forth.
It is an object of the present invention to provide a dosimetry technique for low energy radiations.
It is a particular object of the present invention to provide a dosimetry method for low energy beta radiation with particular emphasis directed toward tritium dosimetry.
Another object of the present invention is to provide a dosimetry method employing the highly sensitive RITAC techniques.
Other objects and advantages of the present invention will become apparent upon reading and consideration of the following description.